Electrodepositing chromium on aluminum



ELECTRODEPOSITING CHROMIUM ON ALUMINUM Clarence W. Forestek, Cleveland,Ohio No Drawing. Application September 5, 1952, Serial No. 308,140

6 Claims. (Cl. 204-38) This invention relates to the electrodepositionofmetals, and in particular relates to a method of electrodepositing anadhesive layer of chromium directly onto an aluminum or other metallicsurface.

In the past, the commercial success of the electroplating of aluminumalloys with chromium, has been predicated upon the establishment of asatisfactory adhesive bond between the chromium and the aluminum oraluminum alloy being plated. The many methods employed to efiectuatethis requisite bond have included both mechanical and chemicaltreatments, all of which have proven commercially unsatisfactory for onereason or another. I

Attempts directed towards the establishment of a mechanical bond betweenthe aluminum alloy and the chromium deposited thereon have included asemi-mechanical pretreatment of the aluminum with a deoxidizing etchingsolution for the purpose of toughening the surface of the same beforesubjecting the alloy to electrodeposition. The bond formed by thismethod lacks the requisite adhesive qualities to make the samecommercially practical.

The most successful bond heretofore attainable has resulted from thechemical treatment of the aluminum alloy by subjecting the same to thezinc immersion treatment after the usual preliminary cleaning treatmentin known manner. In this method, a thin layer of metallic zinc is firstdeposited on the aluminum alloy in the form of a zincate, by immersingthe alloy in a highly alkaline solution of zinc oxide at roomtemperature for a relatively short period of time (30 seconds to 1minute). The zincate covered aluminum alloy is then transferred to anelectrolytic chromium solution operated within a temperature range of65-70 F. After a short (3 to 5 min.) immersion in this bath, thealuminum alloy is then transferred to similar electrolytic chromiumsolution operating at a higher temperature range of 130-140 F. Immersionin this solution for the requisite time period necessary to electroplatethe specified thickness, completes the electrodeposition. In someinstances, the use of a second tank has been avoided by heating thefirst tank from the 63 F. temperature to the 130 F. temperature.

The disadvantages of the zincate method are threefold. First, becausethe bond is predicated upon the provision of a layer of zincate, it ismanifest that if areas of this zincate are prematurely destroyed, as byinadvertent splashing with the chromium electrolytic solution, theresultant chromium plate will be poorly bonded in the aiiected area.Secondly, the actual electroplating of chromium at two differenttemperatures, is disadvantageons because of the cost involved inmaintaining a second tank at a different temperature.

Notwithstanding the foregoing, the third and most serious disadvantagearises by virtue of the dual temperatures employed to effectuate'electrodeposition under the zinca'te method. This increase intemperature, 'while permitting electrodeposition of'an adhering chromiumdeposit, also serves to weaken the same because of the attendant stressplaced upon the chromium deposit by the expansion of the aluminum uponsubjection to the deposited at either temperature will be subjected toab-- normal stresses which can only operate to the detriment of thesame. I

Accordingly, it is one object of this invention to provide a method ofdirectly electrodepositing chromium onto an aluminum alloy at a constanttemperature.

It is a further object of this invention to provide a method ofelectrodepositing chromium onto an aluminum surface wherein thedeposited chromium will not be subjected to any abnormal stresses.

It is a further object of this invention to provide a method ofelectrodepositing chromium directly onto an aluminum alloy, wherein thechromium is ultimately directly adhered to the aluminum alloy. t

It is a further object of this invention to provide a method ofelectroplating chromium directly onto an aluminum alloy featuring anexceptionally strong adhesive bond between the aluminum alloy and thechromium deposit thereon.

It is a further object of this invention to provide a method of directlyelectrodepositing chromium onto an aluminum alloy, the principle ofwhich can be utilized to electrodeposit chromium directly onto othermetals.

It is a still further object of this invention to provide amethod ofdirectly electrodepositing chromium ontoan aluminum alloy whereinaccidental splashing of the part with electrolytic solution will notaffect the overall result of the chromium deposit.

Other objects of the invention will become more apparent uponconsideration of the following brief specification.

Broadly speaking, the invention comprises the discovery that a properlycleaned and pretreated alloy of aluminum can be immersed in aconcentrated electrolytic chromic acid solution of certain compositionand uniform temperature and subjected to variation in current density tocause the electrodeposition of an adhesive layer of chrome directly ontothe aluminum alloy.

In practicing the invention, it is preferred to clean and pretreat thealuminum alloy to be plated in accordance with known methods. platedfirst has any oxide film thereon removed by immersion in a cleaningsolution comprising sulfuric acid or other constituents used throughoutthe trade in this regard. Upon removal from this preliminary cleaningsolution, the part is then immersed in the conventional zinc immersionsolution whereby any remaining oxide film is removed and replaced by arelatively thin layer of metallic zinc. This latter operation, ofrelatvely short. duration (30 sec. to 1 min.) is effectuated inasolution;

having a temperature range of F. to F. At this point, the cleaned andpretreated aluminum alloy is immersed in concentrated electrolyticchrome solution in-.

cluding chromic acid, sulfuric acid and water, and being operated at aconstant temperature falling within thereduced to a constant figurefalling within a range of 14- amps.. per'sq. in., for the duration ofthe plating cycle.

To effectuate the plating operation at a constant tempera-.1

Patented Mar. 27, 1956.

Accordingly, the surface to. be

ture, it has been found necessary to increase the concentration of theelectrolytic solution by increasing the weight per gallon of thechrornic and sulfuric acid in the solution to a point heretoforeconsidered unfeasible in the art of hard chrome plating of an aluminumalloy. Generally speaking, the chromic acid concentration falls within arange of 40 to 60 oz. per gal. of solution, while the sulfuric acidconcentration falls within a range of .40 to .60 oz. per gal. ofsolution. Representative examples showing electrolytic solutions and theproper operating conditions thereof, are listed below. In each case,these examples have produced a highly satisfactory chrome deposit onproperly cleaned and pretreated aluminum .alloy.

Example 1 1 Chromic acid oz. per gal 58 Sulphuric acid do .58 Water tomake 1 gal. Temperature F. 100 Current density (for 1st 2 minutes 7amps. per .sq. in Current density (for duration) do 2 Example 2 1Chromic acid oz. per. gal 54 Sulfuric acid do .54 Water to make 1 gal.Temperature F 96 Current density (for 1st 2 minutes) amps. per sq. in 5Current density (for duration) ..-do... 2

Example 3 1 Chromic acid oz. per gal 46 Sulfuric acid do .46 Water tomake '1 gal. Temperature F 90 Current density (for 1st '1 minute) amps.per sq. in 4 Current density (for duration) do 2 Example 4 1 Chromicacid oz. per gal 60 Sulfuric acid do .60 Water to'make 1 gal.Temperature F 120 Current density (for 1st 2 minutes) amps. per sq. in 6Current density (for duration) do 4 A1} aluminum alloys plated in theseexamples were properly cleaned and pretreated to remove any oxide filmprior to immersion in the baths included in these examples.

.A microsection of each of the above examples clearly evidenced theabsence of any substantial coating of metallic zinc upon completion ofthe plating operation. The only evidence of zinc was found in thepresence of extremely thin layer of zinc-aluminum alloy on the partsplated under Examples 2 and 3. In Examples 1 and 4, there was no traceof zinc in any form. From the foregoing, it is manifest that the zinclayer is not used as a foundation upon which chromium can be deposited.Instead, the concentrated solution that is used would operate to destroythe zinc layer after the same completes its sole function of preventingre-oxidation of the cleaned surfaceof the aluminum alloy. Accordingly,it is manifest that other coatings could be substituted for the zincatecoating described in connection with the pretreatment of the alloy priorto immersion in the new and novel bath, provided that the same served toprevent reox'rdation duringthe interim period preceding immersion intheelectrolytic solution.

'Thus, it can be seen from the preceding descriptionthat there :has beenprovided a new andnovel method of electrodepositing chromium :directlyonto an alnmi-' num alloy. The :methodhasheen shown to. be economicaland novel in that the same features a continuous plating temperaturecoupled with a variation in current density during the plating cycle. Ithas also been shown how the aluminum part is not subjected to expansionresulting in an unnecessary stress being placed on the chromiumelectrodepositcd.

Throughout the specification, reference has been made, in a generalsense, to the terms, aluminum, aluminum alloys and chromium. It is to beunderstood that this application is not restricted to use of purealuminum but is equally applicable to alloys wherein aluminum is thebase or foundation metal. Such products are manufactured commercially byThe Aluminum Corporation of America and are predominately aluminum,although there may be traces of other metals provided therein forspecified purposes. By like token, with reference to the use of the termchromium in the specification, it is to be understood that the chromiumreferred to is the product .of electrodeposition from a chromic acidsolution. It .is to be understood that certain trace impurities may bepresent in the electrodeposit that is generally designated as chromium.

Modificationsof .the out deviating from the spirit thereof or the scopeof the appended claims.

What is claimed is:

l. A method of electrodepositing chromium on the surface .of an aluminummember; comprising the steps of; chemically replacing the oxide film onsaid surface with a protective coating of z'ineate to preventre-oxidation upon contact with the atmosphere; .removing said protectivecoating and simultaneously providing an electrodeposit, of chromium,adhered directly to said aluminum surface by electrolysis, of saidaluminum member at constant temperature, in a concentrated electrolyticsolution including .chromic acid and sulfuric acid, said constanttemperature being between F. and F. said chromic acid being present inconcentrations ranging from 40 to 60 ounces per gallon of solution; andsaid sulfuric acid being present in concentrations ranging from .40 to.60 ounces per gallon of solution.

2. The .method of claim .1 further characterized vby the fact that thecurrent density is varied during said electrolysis.

3. A method of electrodepositing chromium onto an.

minurn member while the same is in said oxide free state, a

to prevent reoxidation'thereof; removing said zincate coat-. ing andsimultaneously providing an electrodeposit of chromium adhered directlyto said oxide free surface of said aluminum member, by subjecting saidzincate coated aluminumimember to electrolysis in an electrolyticsolution comprising chromic acid and sulfuric acid, said chromic acidbeing presentin concentrations ranging from 40 to 60 oz. ,pergnllon ofsolution and said sulfuric acid being present in concentrations rangingfrom .40 to .60 oz. per gallon ofsolution; said electrolytic solutionbeing operated at .a constant temperature falling within the temperaturerange of 90 F. to 120 F.

4. The method of claim 3 further characterized by the fact that thecurrent density falls between 4 to 6 amps. per square inch during theinitial electrolysis and is then lowered to fall within arange of lto 4amps. per square inch during the remainder of said electrolysis.

oxide free surface of said ,alumium member by subjecting said zincatecoated aluminum member to electrolysis in vention may be resorted towith an electrolytic solution that includes chromic acid, inconcentration of 54 ounces per gallon of solution, and sulphuric acid inconcentration of .54 ounce per gallon of solution; said bath beingoperated at a constant temperature of 96 F. 5 6. The method of claim 1,further characterized by the fact that the current density falls between4 to 6 amps. per square inch during the initial electrolysis, and isthen lowered to fall within a range of 1 to 4 amps. during the remainderof said electrolysis. 10

References Cited in the file of this patent UNITED STATES PATENTS1,144,000 Roux June 22, 1915 15 Hewitson May 10, 1927 Moore Aug. 22,1933 Mahlstedt July 24, 1934 Heirnan Jan. 1, 1952 Van der Horst Sept. 1,1953 Passal Dec. 8, 1953 FOREIGN PATENTS Great Britain Apr. 30, 1937Great Britain June 8, 1937 ;OTHER REFERENCES Work et al. Transactions ofthe Electrochemical Society, vol. 59 (1931), pp. 429-435

1. A METHOD OF ELECTRODEPOSITING CHROMIUM ON THE SURFACE OF AN ALUMINUM MEMBER; COMPRISING THE STEPS OF; CHEMICALLY REPLACING THE OXIDE FILM ON SAID SURFACE WITH A PROTECTIVE COATING OF ZINCATE TO PREVENT RE-OXIDATION UPON CONTACT WITH THE ATMOSPHERE; REMOVING SAID PROTECTIVE COATING AND SIMULTANEOUSLY PROVIDING AN ELECTRODEPOSIT OF CHROMIUM, ADHERED DIRECTLY TO SAID ALUMINUM SURFACE BY ELECTROLYSIS, OF SAID ALUMINUM MEMBER AT CONSTANT TEMPERATURE, IN A CONCENTRATED ELECTROLYTIC SOLUTION INCLUDING CHROMIC ACID AND SULFURIC ACID, SAID CONSTANT TEMPERATURE BEING BETWEEN 90* F. AND 120* F. SAID CHROMIC ACID BEING PRESENT IN CONCENTRATIONS RANGING FROM 40 TO 60 OUNCES PER GALLON OF SOLUTION; AND SAID SULFURIC ACID BEING PRESENT IN CONCENTRATIONS RANGING FROM 40 TO 60 OUNCES PER GALLON OF SOLUTION. 